The rapid growth of the subscriber base in Global System for Mobile communication (GSM) has increased the need for increased voice capacity. Therefore mobile network operators and telecom equipment manufacturers have agreed to open a new Release 9 work item in the Third Generation partnership Project (3GPP) standardization. The work item has been named “Voice services over Adaptive Multi-user channels on One Slot” (VAMOS). It is described in 3GPP TSG GERAN GP-081949 “New WID: Voice services over Adaptive Multi-user Orthogonal Sub channels”.
The VAMOS air interface is based upon the concept of Adaptive Symbol Constellation, see 3GPP TSG GERAN GP-081633 Draft TR on Circuit Switched Voice Capacity Evolution for GERAN. Two different mobile station supports are envisaged for VAMOS aware mobile stations:
1. VAMOS aware mobile stations with legacy architecture: These mobile stations are supporting Downlink Advanced Receiver Performance (DARP) phase 1 capability and can operate the new designed training sequences. Radio performance requirements for these mobile stations will be specified with higher priority.2. VAMOS aware mobiles with advanced receiver architectures.
The new modulation introduced in 3GPP TSG GERAN GP-081949 “New WID: Voice services over Adaptive Multi-user Orthogonal Sub channels” employs a time varying signal constellation called adaptive alpha-QPSK. This quaternary constellation is parameterized by a real-valued parameter. This real-valued parameter defines the shape of the signal constellation, and it can change from burst to burst. The real and imaginary parts of the baseband signal are assigned to two users and constitute two sub-channels.
Mobile station architectures based on the DARP phase 1 capability employ so-called Single Antenna Interference Cancellation (SAIC). SAIC mobile stations were designed to demodulate a Gaussian Minimum Shift Keying (GMSK) carrier and suppress GMSK-modulated interference. These mobile stations are typically designed to suppress one dominant interferer. They can be used to suppress the second sub-channel in an alpha-QPSK modulated signal and therefore can be used as receivers for the VAMOS technique.
However, the SAIC methodology does not exploit the fact that when alpha-QPSK modulation is used, the second sub-channel has a very particular structure for example a constant 90 degree phase shift with respect to the first sub-channel. Moreover, a SAIC receiver will experience the second sub-channel as the dominant interference and it will lose effectiveness suppressing additional external interferers.
VAMOS aware mobile stations that exploit knowledge of the two training sequences for the two users and of the shape signal constellation have been proposed, see the international patent application No. PCT/SE2008/051255. When an ordinary QPSK demodulator or a proper modification of a QPSK modulator to deal with alpha-QPSK is used, then joint detection is performed. This type of receiver offers optimal performance whenever the signal is corrupted by Gaussian white noise only. But such a receiver is unable to effectively suppress any external interference.
Hence, there exists a need for an improved receiver. In particular there is a need for a receiver adapted to receive a VAMOS transmitted signal.